The present invention relates to a variable capacity condenser. More particularly, the present invention relates to a variable capacity condenser to be used in a pointer for inputting a coordinate into a computer.
Prior art variable capacity condensers to be used in a pointer are disclosed in Japanese laid-open patent publication Nos. 4-96212 and 5-275283. FIG. 8 illustrates a pointer in the form of an electric pen (hereinafter referred to as a hard-type electric pen) disclosed in Japanese laid-open patent publication No. 4-96212. The electric pen illustrated in FIG. 8 comprises an electric pen casing 801 and a circuit substrate 814. A resonance circuit is provided on the circuit substrate 814 and cooperates with a pointing device to input a coordinate of the electric pen into a computer (not shown). Although not illustrated in FIG. 8, a pointing device, conventionally referred to as a tablet, typically includes a flat input surface having many loop coils arranged in parallel and in every direction. The tablet detects and inputs into a computer a coordinate position as well as a tool force of the electric pen by an electromagnetic exchange between the resonance circuit of the electric pen and a loop coil in the tablet.
The resonance circuit provided on the circuit substrate 814 includes a fixed condenser 813, a variable capacity condenser, and a coil 803. The variable capacity condenser mainly includes a dielectric substance 808, an electrode 809, and a deposition film 816. The electrode 809 is provided on one end face of the dielectric substance 808 by baking. A pin 811 electrically connects the electrode 809 to a terminal in the resonance circuit. The deposit film 816, a second electrode, faces the other end face of the dielectric substance 808. A pin 812 electrically connects the deposit film 816 to another terminal in the resonance circuit.
The variable capacity condenser further includes a spacer 807 and a silicon rubber 815. The spacer 807 is positioned between the other end face of the dielectric substance 808 and the deposit film 816. The silicon rubber 815 is positioned between the deposit film 816 and a core holder 804. As illustrated in FIG. 8, an upper switching case 810 and a lower switching case 805 clip the components of the variable capacity condenser (dielectric substance 808, the electrode 809, the pin 811, the spacer 807, the deposit film 816, the pin 812, and the silicon rubber 815) as well as the core holder 804.
As illustrated in FIG. 8, a core 802 has a pen point protruding outside the electric pen casing 801. As pressure is applied to the pen point, the core 802 slides through a ferrite core 823 having coils 803 wound therearound. The core holder 804 provided at the back end of the core 802 then presses the deposit film 816 via the silicon rubber 815 and decreases the distance between a portion of the deposit film 816 and the other end face of the dielectric substance 808. A portion of the deposit film 816 contacts the other end face of the dielectric substance 808 and the contact area increases as the pressure applied to the pen point increases. As the contact area increases, the air disposed between the deposit film 816 and the other end face of the dielectric substance 808 is expelled. Consequently, the capacity of the variable capacity condenser changes as less air contributes as a dielectric material.
As a result, a resonance frequency of the resonance circuit is deviated because the combined capacity of the variable capacity condenser and the fixed condenser 813 changes. A pointing device (e.g., a tablet) detects this deviant resonance frequency, which depends on the amount of pressure applied to the pent point of the core 802. FIG. 11 illustrates a tool force detected by a pointing device versus a load applied to the pen point of the hard-type electric pen shown in FIG. 8.
In the hard-type electric pen shown in FIG. 8, the ferrite core 823 is a cylindrical core made of L6 member. The ferrite core 823 has an outer diameter of 4 mm, an inner diameter of 2 mm, and a length of 17.5 mm. The coil 803 is a wire of ULAP7/0.07 wound around the ferrite core 823 thirty seven turns. The coil 803 contacts the surface of the ferrite core 823 and is wound around without any gaps between the turns. The coil 803 has an inductance L=26 μH, Q=145 (a frequency of 500 kHz). The dielectric substance 808 is a ceramic with a titanic acid barium system and has an outer diameter of 4 mm and a thickness of 1 mm. Please note that the outer diameter of the dielectric substance 808 and that of the ferrite core 823 are the same. FIG. 8, showing the outer diameter of the dielectric substance 808 being greater than that of the ferrite core 823, is not drawn to scale. The electrode 809 is deposited on the one end face of the dielectric substance 808. The other end face of the dielectric substance 808 is mirror-finished. The silicon rubber 815 has a thickness of 0.4 mm and an outer diameter of 4 mm. As the spacer 807, UPILEX™ of 40 micrometers is used. The spacer 807 is toroidal in shape and has an outer diameter of 4 mm and an inner diameter of 3.3 mm. As the deposit film 816, UPILEX™ of 75 micrometers is used. Nickel chrome having a thickness of 1000 angstroms is deposited on the deposit film 816.
FIG. 7 illustrates a pointer in the form of an electric pen (hereinafter referred to as a soft-type electric pen) disclosed in Japanese laid-open patent publication No. 5-275283. The electric pen shown in FIG. 7 differs from the hard-type electric pen shown in FIG. 8 in its variable capacity condenser. By having a modified variable capacity condenser, the electric pen shown in FIG. 7 reacts to a lighter load.
In the soft-type electric pen, a conductive rubber 706, comprising a mixture of synthetic rubber and conductive particles, replaces the deposit film 816 and the silicon rubber 815 of the hard-type electric pen. In addition, the thickness and inner diameter of a spacer 707 change from those of the spacer 807. Furthermore, instead of a mirror-finished surface of the dielectric substance 808, the other end face of a dielectric substance 708 is a harsh surface ground by a file No. 320. These modifications produce a tool force-load property illustrated in FIG. 10.
A comparison of FIGS. 10 and 11 reveals that the soft-type electric pen reacts to a lighter touch than the hard-type electric pen. For example, FIG. 10 shows that the soft-type electric pen generates a tool force of about 55 when a load of 50 g is applied. For the same applied load of 50 g, however, FIG. 11 shows that the hard-type electric pen generates a tool force of about 30.
In the soft-type electric pen, a ferrite core 723 is a cylindrical core made of L6 member. The ferrite core 723 has an outer diameter of 4 mm, an inner diameter of 2 mm, and a length of 17.5 mm. The coil 703 is a wire of ULAP7/0.07 wound around the ferrite core 723 thirty seven turns. The coil 703 contacts the surface of the ferrite core 723 and is wound around without any gaps between the turns. The coil 703 has an inductance L=26 μH, Q=145 (a frequency of 500 kHz). A dielectric substance 708 is a ceramic with a titanic acid barium system and has an outer diameter of 4 mm and a thickness of 1 mm. Please note that the outer diameter of the dielectric substance 708 and that of the ferrite core 723 are the same. FIG. 7, showing the outer diameter of the dielectric substance 708 being greater than that of the ferrite core 723, is not drawn to scale. An electrode 709 is burned into one end face of the dielectric substance 708. The other end face of the dielectric substance 708 is ground by a file No. 320. As the spacer 707, UPILEX™ of 75 micrometers is used. The spacer 707 is toroidal in shape and has an outer diameter of 4 mm and an inner diameter of 2.4 mm. The conductive rubber 706 has a thickness of 0.4 mm and an outer diameter of 4 mm.
The conductive rubber 706 comprises a mixture of synthetic rubber and carbon particles. The conductive rubber 706 functions as an electrode because the carbon particles are conductive. The surface of the conductive rubber 706 is rough to some extent. As mentioned above, unlike the dielectric substance 808 in the hard-type electric pen, the other end face of the dielectric substance 708 facing the conductive rubber 706 is ground by a file No. 320. Accordingly, a high exfoliation can be achieved after the conductive rubber 706 contacts the other end face of the dielectric substance 708. Moreover, because of the high flexibility of the conductive rubber 706, the capacity of the variable capacity condenser in the soft-type electric pen can be varied with a lighter touch.
The conventional hard-type and soft-type electric pens described above have many components, including common components such as a core holder, an upper switching case, a lower switching case, and pins for two electrodes. Because of these many components, the manufacturing cost of the conventional electric pens is high. Also, the conventional electric pens require many manufacturing and assembly steps. Furthermore, the outer diameter of the conventional electric pens cannot be narrowed beyond a certain size because they have to accommodate the upper and lower switching cases.